Conventional speed indicators for marine vehicles rely on the Pitot principle and include a Pitot tube which projects from the marine vehicle. As the marine vehicle moves over the water surface, water impinges on the open end of the Pitot tube. This creates pressure within the Pitot tube having a magnitude dependant on the speed of the marine vehicle. The pressure created within the Pitot tube operates an analog gauge so that a visual indication of the speed of the marine vehicle is provided.
Unfortunately, speed indicators of this type suffer from disadvantages in that particles in the water such as algae and other debris may cause plugging in the Pitot tube. Plugging in the Pitot tube affects the pressure created therein resulting in inconsistent speed readings being displayed by the analog gauge.
Other types of speed indicators for marine vehicles have also been considered such as those described in U.S. Pat. No. 3,706,224 to Overs and U.S. Pat. No. 4,507,960 to Hufnagel et al. The Overs speed indicator includes a drag impeller mounted within an enclosure installed in the hull of a marine vehicle. The impeller rotates as the marine vehicle travels over the water. Magnets are attached to some of the vanes of the impeller and are detected by a sensor as the impeller rotates. The number of times the magnets are detected by the sensor within a given time frame, is used by a circuit to calculate the speed of the marine vehicle.
The Hufnagel et al speed indicator also includes an impeller within a mounting member received in a recess formed in the bottom of a marine vehicle. Some of the wanes of the impeller have permanent magnets attached to them which are detected by a Hall Effect sensor as the impeller rotates. The number of times the permanent magnets are detected by the sensor within a given time frame is used to determine the speed of the marine vehicle.
Although the Overs and Hufnagel et al speed indicators avoid the need for a Pitot tube, because the impellers in these speed indicators are located within housings, they are still susceptible to plugging which can cause inconsistencies in the speed readings given by the speed indicators. Also, because these designs require the impellers to carry permanent magnets, manufacturing costs become significant. Accordingly, there is a need for an improved speed indicator for marine vehicles.
It is therefore an object of the present invention to obviate or mitigate at least one of the above disadvantages by providing a novel speed indicator for marine vehicles.